I vaguely knew that the Pinata had some kind of pagan origin and I assumed that having a Pinata at a JW party was frowned on by the b0rg. However, despite its blatant pagan origins, apparently I was wrong.
The 22 September, 2003 Awake states that the Aztecs would smash a clay pot full of 'treasures' in honour of the god Huitzilopochtli's birthday. Spanish missionaries simply switched the pot for the Pinata during the Christian conversion of the Aztecs. The same article mentions that the Pinata is also tied closely to Christmas celebrations.
After setting the background, the same article of the Awake says the following: 'While remaining sensitive to the views of others, when considering to use a Pinata at a gathering, the main concern is not what the practice meant hundreds of years ago, but how it is viewed today'. The article in no way condemns the use of a Pinata. Incidently, Huitzilopochtli was the primary god of war.
When discussing birthdays, jw.org's claims the exact opposite. Their main concern revolves completely around what the practice allegedly meant hundreds, or even thousands of years ago. They claim birthdays are tied to magic and folklore and that the bible only mentions birthdays twice, and, both times, bad things happened. Also, no birthday of a faithful servant of God is recorded, conveniently forgetting about Job celebrating his childrens birthdays.
There are very few people - in the western world at least, that still associate birthdays with magic, folklore, or what happened to someone thousands of years ago during a birthday. Birthdays are a naturally occuring event - we cant avoid them. I would argue that using a Pinata, which is tied to the birthday of an Aztec god of war - and Christmas in more modern times, should be far more offensive to JW's than birthdays.